Certain key aspects of hepadnavirus biology, including the mechanisms controlling resolution of an infection, and infection progressing to neoplastic transformation, have not yet been elucidated. For example, (i) we don't understand how transient infections can resolve rapidly even after infection of the entire hepatocyte population, or what may initiate the resolution of chronic infections after years or even decades of virus production. (ii) We do not yet know2 the process ov virus quasi-species evolution during chronic infections and its relationship to disease outcome, or the outcome of antiviral therapy. (iii) The cellular changes leading to hepatocellular carcinoma during a hepadnavirus infection have not yet been described. (iv) We still don't know the in vivo role of the X gene product, which is required for infection, but not for virus production by transfected cell lines. Without this knowledge, the possibilities for treating and even curing chronic infections is limited. However, recent technical advances, and refinements of older procedures, now make it feasible to begin to address these problems in greater detail. The specific aims of our research are therefore to use woodchuck hepatitis virus. 1) To determine the mechanisms of virus loss during recovery from a transient infection. Experiments will be carried out to determine the kinetics and cumulative loss of hepatocytes during recovery from a transient infection, and possible mechanisms by which this may occur. 2) To determine how viruses evolve during the course of a chronic infection. In the interim, mechanisms underlying more conventional antiviral approaches are being analyzed. The proposed experiments focus on the mechanism of virus evolution with a clear practical significance, the outgrowth of drug resistant populations during therapy with antiviral nucleosides. 3) To characterize pre-neoplastic lesions. A long-term goal of antiviral therapy is to reverse neoplastic transformation leading to HCC. Studies will therefore be undertaken to identify molecular markers and/or determinants of this progression, as a basis for evaluation neoplastic disease progression in chronic hepatitis. 4) To determine the role of the X gene product in initiation/maintenance of an infection in cell culture and in the woodchuck liver. There is an absolute requirement for the X gene product of viral reproduction in vivo, making it an ideal therapeutic target. Experiments are described to determine where in the virus life cycle X is required.